This is the default Firefox behavior designed for security .The assumption is probably that most web sites don’t know what and where are you local files (including UNC paths).
This could be turned off in firefox:
- type “about:config” in the address bar and accept “i’ll be careful”
- find “security.checkloaduri” in older versions or “security.fileuri.strict_origin_policy” in newer versions of firefox and change the value to “false”
- restart firefox
That should do it for you. You have more information here: